Recently, a carbon dioxide analyzing system has been developed for detecting live insects in commodity samples. A commodity sample is introduced into a commodity test chamber (CTC), and the system is closed to the environment for a given period of time. During this incubation time, any insects in the sample will respire or otherwise give off carbon dioxide. After this given period of time, the system is opened to the ambient air, and the carbon dioxide in the test chamber is forced under pressure or vacuum to a sample cell. The carbon dioxide level in this sample cell is then compared with a reference level carbon dioxide in a reference cell. Infestation will be indicated in the commodity sample, when the two levels do not coincide.
The above system is described in the literature in the following articles: "Detection of Hidden Insects" by William A. Bruce and Marion W. Street, Jr., Department of Agriculture, Washington, D.C., 18 June, 1975, (PB 248 002); Department of Agriculture Report (ARS-S-85) by William A. Bruce and Marion W. Street, Jr., March, 1976; and "CO.sub.2 Analyzer Detects Insects Hidden in Foods" by William A. Bruce and Marion W. Street, Jr., Food Engineering, February, 1976.
Reference is additionally made to U.S. Pat. No. 3,963,927, issued June 15, 1976.
While this prior system has made a major contribution in the art of insect detection, certain shortcomings have been noted to exist. This system has been found to give results which are sometimes neither accurate nor reproducible, as the system is open to the ambient surroundings during the transfer of the carbon dioxide from the commodity test chamber (CTC) to the sample cell. The ambient air is sucked into the system as the carbon dioxide in the test chamber is pumped to the sample cell. The ambient air will normally contain the same level of carbon dioxide as in the reference cell, which was open to ambient air prior to incubation. If such is the case, the system will give accurate results. However, this is not always true. Ambient conditions may change during incubation, as particularly so when people breathe or smoke near the system inlet, so as to cause a higher carbon dioxide concentration to exist in the air sucked into the sample cell. Thus, false readings often result.
The reliability of the system is dramatically impaired by these extraneous concentrations of carbon dioxide, particularly since very low levels of carbon dioxide are to be measured against a high reference level. Thus, a person breathing near the system can generate enough carbon dioxide to give an indication of infestation, when in actuality no insects are present in the sample.
A single insect may generate only several parts per million (ppm) of carbon dioxide in a few minutes. This minute amount of carbon dioxide must be measured against an ambient background of approximately 270-300 ppm. Thus, even the slightest change in ambient surroundings of the system will greatly influence the accuracy and reliability of the analysis.
The present invention seeks to overcome the above drawbacks of the prior art by providing a controlled background level of carbon dioxide in both the sample and reference cells. This controlled environment ensures that any change in the level of carbon dioxide in the sample cell is due entirely to infestation, and not to any changes in the ambient surroundings. The invention contemplates providing a closed-loop system to avoid extraneous carbon dioxide from entering the system and, furthermore, supplying each of the sample and reference cells with a controlled amount of background carbon dioxide, whereby the reference, or background, level of carbon dioxide remains constant throughout the analysis.
It is known to have closed loops in other types of analyzing systems, such as described in "A Rapid and Specific Method for the Estimation of Glucose Using an Oxygen Electrode and Simple Differentiating Circuit" by H. L. J. Makin, P. J. Warren and J. D. Edridge, Clinica Chimica Acta, 84 (1978), pages 137-143. However, such prior systems have used the closed loop as a means to save the enzyme for reuse, and not to precisely control the system parameters during the analysis process.